Engineered fluoride-coated elements for laser systems

ABSTRACT

The invention is directed to elements having fluoride coated surfaces having multiple layers of fluoride material coatings for use in laser systems, and in particular in laser systems operating at wavelength &lt;200 nm. In a particular embodiment the invention is directed to highly reflective mirrors for use in wavelengths &lt;200 nm laser systems. The invention describes the mirrors and a method of making them that utilizes a plurality of periods of fluoride coatings, each period comprising one layer a high refractive index fluoride material and one layer low refractive index fluoride material, and additionally at least one layer of an amorphous silica material. The silica material can be inserted between each period, inserted between a stack consisting of a plurality of periods, and, optionally, can also be applied as the final layer of the finished element to protect the element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 110(e) of U.S. Provisional Application Ser. No. 60/904,132 filed on Feb. 28, 2007, the content of which is relied upon and incorporated herein by reference in its entirety.

FIELD

The invention is directed to fluoride coated surfaces and elements having multiple layers of fluoride material coatings for use in laser systems. In particular, the invention is directed to surfaces, for example, highly reflective mirrors for use with 193 nm lasers that have multiple layers of coatings of fluoride materials.

BACKGROUND

ArF excimer lasers are the illumination sources of choice for the microlithographic industry. The industry constantly demands more performance from excimer laser sources. As a result, greater demands are constantly placed on excimer laser optical components, for example, the highly reflective mirrors that are used in 193 nm wavelength excimer lasers that operate at high repetition rates. These highly reflective mirrors are typically made using at least one high refractive index material and one low refractive index material. Among the very limited number of materials that can be used for such mirrors, GdF₃ and LaF₃ are considered as high refractive index materials and MgF₂ and AlF₃ are the low refractive index materials that are used for wavelengths below 200 nm. [see D. Ristau et al., “Ultraviolet optical and microstructural properties of MgF₂ and LaF₃ coating deposited by ion-beam sputtering and boat and electron-beam evaporation”, Applied Optics 41, 3196-3204 (2002); R. Thielsch et al., “Development of mechanical stress in fluoride multi-layers for UV-applications”, Proc. SPIE 5250, 127-136 (2004); C. C. Lee et al., “Characterization of AlF₃ thin films at 193 nm by thermal evaporation”, Applied Optics 44, 7333-7338 (2005); R. Thielsch et al, “Optical, structural and mechanical properties of gadolinium tri-fluoride thin films grown on amorphous substrates”, Proc. SPIE 5963, 5963001-12 (2005); and Jue Wang and Robert L. Maier, “Nano-structure of GdF₃ thin film evaluated by variable angle spectroscopic ellipsometry”, Proc. SPIE 6321, p6321071-10 (2006)]. At the present time there is renewed research interest in wide band-gap fluoride thin films for ArF laser optics applications. The application of energetic deposition processes, such as plasma ion-assisted deposition (PIAD), ion assisted deposition (LAD) and ion beam sputtering (IBS), are restricted for fluoride materials because of the nature of fluoride materials. As a result, the industry has accepted thermal resistance evaporation for fluoride film deposition without introducing fluorine depletion. However, when thermal resistance evaporation is used as the film deposition method, the resulting fluoride film packing density is low (that is, it is porous) and the film structure is inhomogeneous. Neither of these is desirable because a porous structure can harbor environmental contamination and increases scatter losses. Various approaches have been applied to improve fluoride film structure including substrate temperature and deposition rate. Recently, the effect of substrate crystal orientation has also been taken into account, but no significant improvements have been reported. [see Y. Taki and K. Muramatsu, “Hetero-epitaxial growth and optical properties of LaF₃ on CaF₂”, Thin Solid Films 420-421, 30-37 (2002), and US patent 02/276,670 A1 to Y. Taki et al., titled “OPTICAL ELEMENT EQUIPPED WITH LANTHANUM FLUORIDE FILM”].

Another problem arises from the fact that many periods of high index and low index layers (one period equals one high and one low refractive index layer) are required in order to get high reflectivity at 193 nm. However, the surface/interface roughness and inhomogeneity increase as number of layers and the overall thickness increases. The control of the multilayer fluoride film structure is critical for achieving high reflectivity at 193 nm. In addition to their use in microlithography, fluoride coated mirrors are also required for those areas where ArF excimer laser have other, non-lithographic application, for example minimal invasive brain-, vascular- and eye surgery; ultra-precision machining & measurement; large-scale integrated electronic devices; and components for communication. In view of the problems extant with the present fluoride coated elements, for example mirrors, that are used in below 200 nm, and particularly in 193 nm, lithography, it is desirable to have fluoride coated elements that overcome these problems. In addition to mirrors, the invention can also be applied to beamsplitters, prisms, lenses, output couplers and similar elements used in <200 nm laser systems.

SUMMARY

The invention is directed to elements for use in <200 nm laser systems, the element comprising a substrate; one or a plurality of periods of fluoride coating materials, each of said periods comprising at least one layer of a high refractive index fluoride material and at least one layer of a low refractive fluoride material; and at least one layer of an amorphous SiO₂ material selected from the group consisting of amorphous silica, amorphous F-doped SiO₂, amorphous Al₃O₂-doped SiO₂, and amorphous N-doped SiO₂. In accordance with the invention the amorphous SiO₂ material can be inserted after each period of the fluoride coating material or after a stack consisting of a plurality of periods of fluoride coating material. Optionally, a layer of the amorphous SiO₂ material can be applied to the substrate prior to the application of the first period of fluoride coating materials. The fluoride coating materials are metal fluoride materials having a high refractive index and low refractive index, and these materials are applied in alternating layers to the substrate or amorphous SiO₂ coated substrate. In some embodiments the high refractive index fluoride material has an index in the range of 1.65 to 1.75 and said low refractive fluoride material has an index in the range of 1.35 to 1.45. Substrates that can be used for the elements are glass and glass-ceramic substrates; alkaline metal fluoride single crystal substrates; metallic substrates, for example without limitation, substrates made from aluminum, titanium, and other metals known in the art that are know to be resistant to deterioration in a 200 nm laser environment such as found in ArF and F₂ lasers; and other materials resistant to deterioration in a 200 nm laser environment such as found in ArF and F₂ lasers, for example silicon nitride (Si₃N₄). The amorphous SiO₂ material includes amorphous SiO₂ itself and doped amorphous SiO₂ materials such as silica materials doped with fluorine (F), silica materials doped with Al₃O₂, and silica materials doped with nitrogen (N), fluoride (F) and Al₂O₃. Each period of fluoride coating has a thickness in the range of 50 nm to 90 nm, and within the period the high refractive index material has a coating in the range of 20 nm to 40 nm, and the low refractive index material has a thickness in the range of 30 nm to 5 nm. The SiO₂ layer applied to the substrate before coating (optional), or after a period or stack or periods, has a thickness in the range of 5 nm to 75 nm. The final layer of SiO₂ applied to element after the deposition of all the fluoride coating periods has a thickness in the range of 10 nm to 150 nm.

The invention is further directed to a method for preparing fluoride coated elements and in particular highly reflective fluoride coated mirrors for use in <200 nm laser systems. The method has at least the steps of providing a substrate; coating the substrate with one or a plurality of periods of fluoride coating materials using an energetic deposition technique, each such period having at least one layer of a high refractive index fluoride material and at least one layer of a low refractive index fluoride coating material; and further coating using an amorphous SiO₂ material including amorphous SiO₂ material itself and doped amorphous SiO₂ materials. In one embodiment of the method the SiO₂ is applied as a layer after each fluoride coating period. In another embodiment of the method the SiO₂ layer is applied after a stack of a plurality of fluoride coating material periods. In another embodiment of the method a SiO₂ layer is applied to the substrate prior to the applying the first period of fluoride coating materials. Energetic deposition techniques that can be used in practicing the invention include PIAD (plasma ion-assisted deposition), IAD (ion-assisted deposition), and IBS (ion beam sputtering).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the refractive index depth profile of a GdF₃ film grown on CaF₂ (111).

FIGS. 2A and 2B are AFM images of GdF₃ single layer grown on a CaF₂ (111) single crystal.

FIG. 3 is a graph illustrating the absorbance of a GdF₃ film at 193 nm as a function of air exposure time.

FIG. 4 is a schematic diagram illustrating of a standard fluoride mirror having high and low refractive index materials coated on a substrate.

FIG. 5 is a graph illustrating the surface roughness of standard fluoride mirrors as a function of stack period.

FIG. 6 is a graph illustrating the scatter loss of standard fluoride mirrors as a function of stack period.

FIG. 7 is a graph illustrating the reflectance of standard fluoride mirrors as a function of stack period.

FIGS. 8A and 8B are AFM images comparing the PIAD smoothed SiO₂ film of FIG. 8A with the uncoated SiO₂ substrate of 8B.

FIG. 9 is a schematic diagram illustrating the surface and interface of an engineered fluoride mirror.

FIG. 10 is a schematic diagram illustrating the surface and interface of an engineered fluoride-enhanced-oxide mirror.

FIG. 11 is a schematic diagram illustrating the surface and interface engineered fluoride-enhanced-Al mirror.

DETAILED DESCRIPTION

As used herein, the terms “fluoride coated mirrors” and “fluoride mirrors” include mirrors in which the fluoride coating is applied to substrates such as alkaline earth metal fluoride single crystal materials (single crystals of CaF₂, BaF₂, SrF₂, BaF₂), glass materials (for example SiO₂, HPSF® (Corning Incorporated), BK7™ and SF10™ (Schott Glass)), metallic materials (for example, aluminum, titanium) and other materials (for example, Si, Si₃N₄). Also has used herein, the term “period” as applied to fluoride coatings means one high refractive index layer and one low refractive index layer. The term “stack” as used herein means two or more periods of fluoride materials coated on the substrate which lie between the substrate and the SiO₂ films of the invention or between two SiO₂ films.

In wavelengths <200 nm laser systems, for example, ArF laser systems which operate at 193 nm, scatter and contamination are the main factors that degrade the performance of fluoride mirrors and fluoride enhanced mirrors. Fluorine depletion generates significant absorption at 193 nm if advanced energetic deposition processes are used to improve fluoride film structure and reduce scatter loss. Among the problems associated with standard fluoride mirrors, including fluoride-enhanced-oxide mirrors and fluoride-enhanced-Al mirrors are:

-   -   (1) it is difficult to obtain a reflectivity over 97% at 193 nm,     -   (2) the reflectivity is environmental sensitive,     -   (3) the porous structure of fluoride films harbors         contamination, leading to absorption at 193 nm, and     -   (4) laser durability is low         The present invention overcomes these problems by insertion of         amorphous silica (SiO₂) layers between a period or stack of         periods of the high and low refractive index materials that are         use to coat the mirror blanks (substrates). In general, the         invention is directed to using a PIAD smoothed SiO₂ layer is         periodically inserted into fluoride stacks and also use SiO₂ as         the top, capping or final layer. More specifically, the         invention relates to:     -   (1) inserting amorphous SiO₂ layer to eliminate the growth of         inhomogeneous fluoride film structure,     -   (2) using PIAD to smooth the amorphous SiO₂ layer surface,     -   (3) continuously growing fluoride films on the smoothed SiO₂         surface, and     -   (4) using the dense smoothed SiO₂ as capping layer to reduce the         fluoride exposed surface area by a factor of ˜10⁻⁵ and thus to         seal the engineered fluoride mirror.

[1]. The Surface Roughness of a Fluoride Single Layer

Standard fluoride mirrors comprise multiple layers of fluoride materials; in particular, alternating layers of a high refractive index material and a low refractive index material. The fluoride film growth mechanism was investigated using a GdF₃ single layer as an example, and the results indicate that the layer has an inhomogeneous structure and a rough surface. FIG. 1 shows the refractive index (at 193 nm) depth profile of a GdF₃ film grown on CaF₂ (111) surface. The refractive index is proportional to film packing density. In general for the GdF₃ film, high refractive index originates from a dense film, whereas low refractive index corresponds to a porous film structure. As can be seen in FIG. 1, at the beginning of the GdF₃ film formation a dense thin layer is formed on the substrate leading to a refractive index of 1.738. As the film thickness increases, the growth mechanisms of columnar and polycrystalline microstructure may introduce gaps between crystal grains. As a consequence, the film density decreases as the layer thickness accumulates. At the end of film growth the refractive index has further dropped to 1.62, corresponding to a mean porosity of 15.8%. A refractive index of 1.35 represents a 3.5 nm surface roughness layer in FIG. 1. High refractive index fluoride materials have an index of refraction n in the range of 1.65 to 1.75, and low refractive index fluoride material has an index on the range of 1.35 to 1.45.

FIGS. 2A and 2B exhibits AFM (atomic force microscopy) images over 1μ×1μ and 5μ×5μ scanning areas of the GdF₃ layer, respectively. The grain and the pore sizes shown in FIGS. 2A and 2B range from 300 nm to 350 mm. The AFM images clearly reveal the nano-porous morphology of GdF₃ film growth on a CaF₂ (111) surface. As can be seen in the 1μ×1μ image of FIG. 2A, there are some gaps between the accumulated dense grains, leading to the formation of porous structure. By increasing AFM scanning size to 5μ×5μ as shown in FIG. 2B, the porous network is obvious on the film growth plane. As predicted by ellipsometric modeling, the inhomogeneity of the GdF₃ film is a result of film porosity changes during growth. The randomly distributed porous structure with relatively high internal surface area may connect to each other to some degree and may harbor environmental contamination.

FIG. 3 shows the absorbance of the GdF₃ film at 193 nm as a function of laboratory ambient exposure (air exposure). The film absorbance increases over exposure time to air. In summary, fluoride films are inhomogeneous and porous in general. As layer thickness increases, film surface roughness increases. The porous film structure and rough surface lead to high absorption and scatter loss at 193 nm wavelength.

[2]. Surface and Interface of Standard Fluoride Mirror

Based on our experimental results, the surface roughness of standard fluoride mirror, R_(m), can be described by Equation (1),

R _(m) =αp+βR _(s)  (1)

where α and β are fluoride material and deposition process related parameters, Rs is the surface roughness of the substrate, and p is the stack period. A stack period is defined as a combination of low refractive index and high refractive index layers. Parameter α is related to the high and the low refractive fluoride layers used for the period such as GdF₃/MgF₂, LaF₃/MgF₂, GdF₃/AlF₃ and LaF₃/AlF₃, the deposition rate and substrate temperature for each material. Parameter β is dominated by the substrate material properties and surface finishing condition. Using standard fluoride mirrors, for example, at a normal angle of incidence, i.e., 0°, the fluoride mirrors compromise stacks of high and low refractive index layers as represented by Formula (2):

Sub_H(LH)^(p)  (2)

where H and L corresponds to a quarter-wave high index GdF₃ and a quarter-wave low index AlF₃, respectively, and p is a stack period. A schematic of the mirror is shown in FIG. 4. [Note: In all the figures, the substrate is numbered as 20; H is numbered as 30; L is numbered as 40, the SiO₂ layer 2M is numbered as 50.]

FIG. 5 is a graph showing the surface roughness of standard fluoride mirrors as a function of stack period. The surface roughness is linearly proportional to the stack period as described in formula (1). That is, the more periods a standard mirror contains the rougher the surface becomes. FIG. 6 shows surface and interfacial scatter loss of fluoride mirrors as a function of stack period. Generally, a large number of stack periods (p>16) are required in order to achieve high reflectivity in the final mirror product. As can be seen from FIG. 6, scatter loss increases slowly when the stack period is a small number. However, FIG. 6 shows that the slope of the curve increases along with an increasing number of stack periods. In other words, scatter loss increases faster than the additional reflectivity gain due to adding more stack periods into standard fluoride mirror. As a result, there is an optimized number of stack periods which offers the highest reflectivity when one takes into account of scatter loss.

FIG. 7 shows reflectance of standard fluoride mirror as a function of stack period. According to the design calculations (design curve is numeral 80), the reflectance as a function of stack period can be separated into 3 zones which are:

1. a fast increasing region (up to 6 periods),

2. a slow increasing region (more than 16 periods) and

3. transition region from the fast increase to the slow increase.

The achievable reflectance (numeral 82), which also has 3 zones, is plotted in the same figure for comparison. The fast increasing region of the achievable reflectance is almost the same as that of the design reflectance. The transition zone for the achievable reflectance is very similar to the design, but there is a small separation when the stack period number is located at the high end of the zone. The main difference between the achievable reflectance and the design reflectance is located in the region where the period number is greater than 16. Instead of a slow increase of reflectance as shown for the design reflectance, a slow decrease of reflectance appears in the high period region for the achievable reflectance because of scatter loss. Consequently, in order to successfully make a high reflectance fluoride mirror it is necessary to eliminate surface and interface roughness. As discussed above, energetic deposition techniques cannot be used to make dense smooth fluoride films without generating fluorine depletion. The invention describes high reflectance fluoride mirrors which utilize SiO₂ based oxide layers to smooth-out fluoride films to achieve high reflectivity.

[3]. Smoothed Amorphous SiO₂ by Energetic Deposition

In accordance with the invention, an amorphous but dense and smooth, SiO₂ film is inserted into a fluoride stack by means of energetic deposition. Dense smooth SiO₂ films can be deposited by PIAD, LAD and IBS. Here PIAD deposited SiO₂ is used as an example. FIG. 8A is an AFM image of PIAD smoothed SiO₂ film on a SiO₂ substrate. The surface roughness of the uncoated substrate is 0.35 nm. After depositing a 200 nm SiO₂ film, the surface roughness has been reduced to 0.29 nm. The result shows that the smoothed SiO₂ film reduces roughness of an uncoated substrate. FIG. 8B illustrates how a SiO₂ film can be also used to improve, that is, decrease, the surface smoothness of a coated substrate. In accordance with the invention, at least one PIAD deposited SiO₂ layer has been inserted into standard fluoride mirror to protect fluoride films from plasma ion direct bombardment, and to smooth out the fluoride film's accumulated rough structure. Fluoride stacks can be continuously deposited on the smoothed SiO₂ film layer. In preferred embodiments the final layer is a SiO₂ film layer. In FIG. 8B, the surface roughness is 0.29 nm after application of the SiO₂ film. Before application of the SiO₂ film the surface roughness was 0.35 nm.

[4]. Surface and Interface Engineered Fluoride Mirror

Surface and interface engineered fluoride mirrors can be described by Formula (3):

Sub _H(LH)^(i)2M(LH)^(j)2M . . . (LH)^(k)2M  (3)

where 2M represents a half-wave SiO₂ layer; H and L are high index and low index fluoride layers, respectively; and i, j . . . and k are stack periods. According to Equation (3), a SiO₂ layer is inserted into standard fluoride mirror every i, j . . . k stack periods. FIG. 9 shows a schematic of surface and interfacial engineered fluoride mirror as represented by Formula (3). Table 1 lists a comparison of standard and surface/interface engineered fluoride mirrors with a stack period of 21, where 3 nm surface and interface roughness is used for the engineered mirror.

TABLE 1 Comparison of standard and surface/interface engineered mirrors Standard Engineered mirror mirror Scatter loss (%) 5.69 1.16 Reflectance design (%) 99.96 99.57 Reflectance achievable (%) 94.27 98.41

Although the designed reflectance of the standard mirror (99.96%) is higher than the engineered mirror (99.57%), the scatter loss of the standard mirror is 4.9 times greater than that of the engineered mirror. The final achievable reflectance is 94.27% and 98.41% for the standard and the engineered mirrors, respectively. In some embodiments of the invention the top layer of the engineered mirror ends with a dense SiO₂ layer as shown in formula (3). This top layer of SiO₂ seals and smoothes the porous structure as is shown in the AFM images of FIGS. 2A and 2B. In addition, as a result of applying a dense SiO₂ film as described herein, the risk for environmental contamination penetrating into the porous fluoride structure is eliminated. This is a further difference between the surface/interface engineered mirrors of the invention as compared to the standard fluoride mirror which can easily be contaminated due to porosity as is illustrated by GdF₃ single layer data shown in FIG. 3.

[5]. Surface/Interface Engineered Fluoride-Enhanced-Oxide Mirror

The invention, in another embodiment, is also directed to fluoride-enhanced-oxide mirrors in which the fluoride enhanced stacks are smoothed by inserting PIAD deposited SiO₂ layers in a Formula (4)

Sub_H₀(L₀H₀)^(i)(LH)^(j)2M . . . (LH)^(k)2M  (4)

or Formula (5)

Sub_(H₀L₀)^(i)H(LH)^(j)2M . . . H(LH)^(k)2M  (5)

where H₀ and L₀ corresponds to a quarter-wave high index Al₂O₃ and a quarter-wave low index SiO₂, respectively; 2M represents a half-wave SiO₂ layer; and H and L are high index and low index fluoride layers, respectively. A schematic of surface and interface engineered fluoride-enhanced-oxide mirror is shown in FIG. 10 where H₀ is numeral 32; L₀ is numeral 42; and H, L and 2M are represented by numerals as previously indicated.

[6]. Surface/Interface Engineered Fluoride-Enhanced-Aluminum Mirror

The invention, in another embodiment, is also directed to fluoride-enhanced-aluminum mirror, in which the fluoride enhanced stacks are smoothed by inserting PIAD deposited SiO₂ layers as represented by Formula (6):

Sub_A(LH)^(j)2M . . . (LH)^(k)2M  (6)

or Formula (7)

Sub_AH(LH)^(j)2M . . . (LH)^(k)2M  (7)

where A is a thickness aluminum (Al) layer; 2M represents a half-wave SiO₂ layer; and H and L are high index and low index fluoride layers, respectively. A schematic of surface and interface engineered fluoride-enhanced-Al mirror is shown in FIG. 11, the aluminum layer (Al) being represented by numeral 70; and H, L and 2M are represented by numerals as previously indicated.

To summarize the characteristics of the invention:

-   -   H can be any high refractive index fluoride material, for         example, GdF₃, LaF₃, and other high refractive index metal         fluoride materials known in the art.     -   L can be any low refractive index fluoride material, for         example, AlF₃, MgF₂, CaF₂ and other low refractive metal         fluoride materials known in the art     -   SiO₂ based layer can be SiO₂ or modified SiO₂, for example,         F-doped SiO₂, N-doped SiO₂, and Al₂O₃-doped SiO₂.     -   Energetic smoothing technique that can be used in practicing the         invention can be PIAD (plasma ion-assisted deposition), LAD (ion         assisted deposition), IBS (ion beam sputtering), and similar         energetic techniques known in the art as being useful for the         deposition of the oxide materials and particularly oxide         materials.     -   The layer order of H and L in Formulas (3)-(7) can be changed.         For example, the order in Formula (3) may change from

Sub_H(LH)^(i)2M(LH)^(j)2M . . . (LH)^(k)2M  (3)

to

Sub_(HL)^(i)2MH(LH)^(j)2M . . . (LH)^(k)2M  (8)

or

Sub_(LH)^(i)2M(LH)^(j)2M . . . (LH)^(k)2M  (9)

-   -   Formulas 4-7 may similarly be changed.     -   Changing the optical thickness in Formulas (3)-(7) enables one         to apply the invention to S-polarization or P-polarization         mirrors at high angle of incidence at 193 nm. Typically the         thickness of the high refractive index layer is in the range of         20 to 35 nm, and the thickness of the low refractive index         material is in the range of 30 to 45 nm. The thickness of the         SiO₂ film inserted, in accordance with the invention, after a         period or a stack of periods is typically in the range of 5 to         75 nm. The thickness can be changed to 22 to 39 nm for the high         refractive index material and 36 to 54 nm for the low refractive         index material for applications to S-polarization or         P-polarization mirrors at high angle of incidence at 193 nm.         Changing optical thickness in Equations (3)-(7) also enables one         to apply the invention to other DUV laser wavelengths, for         example, other <200 nm laser systems that operate at such <200         nm wavelengths such as 157 nm, 198.5 nm.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. 

1. An element for use in wavelengths <200 nm laser systems, said element comprising: a substrate, one or a plurality of periods of fluoride coating materials, each of said periods comprising at least one layer of a high refractive index fluoride material and at least one layer of a low refractive fluoride material, and at least one layer of an amorphous SiO₂ based material selected from the group consisting of silica, F-doped SiO₂, N-doped SiO₂ and Al₂O₃-doped SiO₂ applied to on top of at least one of said periods of fluoride coating materials. wherein said high refractive index fluoride material has an index in the range of 1.65 to 1.75 and said low refractive fluoride material has an index in the range of 1.35 to 1.45; and and, optionally, a coating of an amorphous SiO₂ material between the substrate and the first period of fluoride coating, the optional amorphous SiO₂ material, when applied to said substrate, being in addition to the at least one layer of an amorphous SiO₂ based material.
 2. The element according to claim 1, wherein the number of periods of fluoride coating materials is greater than 1, and the at least one layer of an amorphous SiO₂ material is inserted between at least two periods of the number of periods greater than 1, and the last coating material applied to said element is a layer of a SiO₂ material selected from the group consisting of silica, F-doped SiO₂, N-doped SiO₂ and Al₂O₃-doped SiO₂.
 3. The element according to claim 1, wherein the thickness of each period of fluoride coating is in the range of 50 nm to 90 nm.
 4. The element according to claim 1, wherein the thickness of the high refractive index layer within each period of fluoride coating is in the range of 20 nm to 40 nm.
 5. The element according to claim 1, wherein the thickness of the low refractive index layer within each period of fluoride coating is in the range of 30 nm to 50 nm.
 6. The element according to claim 1, wherein the thickness of the optional SiO₂ layer applied between the substrate and the first period is in the range of 5 nm to 75 nm.
 7. The element according to claim 2, wherein the thickness of the SiO₂ layer inserted between the periods of high and low refractive index materials is in the range of 5 nm to 75 nm.
 8. The element according to claim 2, wherein the last coating material applied to said element is a layer of a SiO₂ material having a thickness in the range of 10 nm to 150 nm, said material being selected from the group consisting of silica, F-doped SiO₂, N-doped SiO₂ and Al₂O₃-doped SiO₂.
 9. The element according to claim 1, wherein said element is selected from the group consisting of reflective mirrors, beamsplitters, prisms, lenses, and output couplers.
 10. The element according to claim 1, wherein the element is a reflective mirror.
 11. The element according to claim 1, wherein the high refractive index material is selected from the group consisting of GdF₃ and LaF₃.
 12. The element according to claim 1, wherein the low refractive index material is selected from the group consisting of MgF₂, CaF₂ and AlF₃.
 13. A reflective mirror element for use in wavelengths <200 nm laser systems, said element comprising: a substrate, a plurality of periods of fluoride coating materials, each of said periods comprising at least one layer of a high refractive index fluoride material and at least one layer of a low refractive fluoride material, and a plurality of layers of an amorphous SiO₂ based material selected from the group consisting of silica, F-doped SiO₂, N-doped SiO₂ and Al₂O₃-doped SiO₂, wherein said layers of amorphous SiO₂ are inserted between periods of said fluoride coating materials or between a stack of a plurality said periods, and, optionally, a coating of an amorphous SiO₂ material between said substrate and the first period of high refractive index fluoride material and low refractive fluoride material; wherein the thickness of the SiO₂ material between the substrate and the first period and the thickness between the periods is in the range of 5 nm to 75 nm.
 14. The reflective mirror element according to claim 11, wherein the last coating on said mirror element is a layer of an amorphous SiO₂ material, said layer having a thickness in the range of 10 nm to 150 nm.
 15. A method for making a fluoride coated element suitable for use <200 nm laser systems, said method comprising the steps of: providing a substrate; coating, using an energetic deposition technique, the substrate with one or a plurality of periods of fluoride coating materials using an energetic deposition technique, and further coating, using an energetic deposition technique, with an amorphous SiO₂ material to thereby form a fluoride coated element suitable for use in <200 nm laser systems; wherein coating the substrate with one or a plurality of periods of fluoride coating materials means coating such that each period comprises a layer of a high refractive index fluoride coating material and a layer of a low refractive index coating material.
 16. The method according to claim 15, wherein said method further comprises coating said substrate with a layer of an amorphous SiO₂ material prior to the application of the first period of fluoride coating materials.
 17. The method according to claim 15, wherein coating using an energetic deposition technique comprises coating using deposition techniques selected form the group consisting of plasma ion-assisted deposition, ion-assisted deposition and ion beam sputtering.
 18. The method according to claim 15, wherein the method further comprises coating said element with a final layer of an amorphous silica after the coating with a plurality of fluoride materials is completed.
 19. The method according to claim 15, wherein coating with said amorphous silica material means coating with a silica material selected from the group consisting of silica, F-doped SiO₂, N-doped SiO₂ and Al₂O₃-doped SiO₂. 